1. Field of the Invention
This invention relates to flueric partial pressure sensors and is more particularly, but not exclusively, concerned with a flueric partial pressure sensor for sensing the partial pressure of oxygen in oxygen enriched air in varying or variable ambient pressure conditions.
2. Description of the Prior Art
It is necessary for a certain minimum oxygen pressure to exist in the lungs for the oxygen to be able to diffuse through the lung tissue and pass to the haemoglobin or red corpuscles in the blood. However, the partial pressure of oxygen in the atmosphere falls off with altitude in direct proportion to the absolute pressure value. This problem must be overcome in aircraft breathing systems so as to provide aircrew with breathing air having a substantially constant partial pressure of oxygen value. In order to maintain a sea level oxygen partial pressure value of 160 mm Hg the level of oxygen concentration in the breathing air must increase from 21% by volume in air at sea level to about 65% by volume in air at 9000 meters (30000 feet).
EP-A-No. 0,036,285 and corresponding U.S. Pat. No. 4,407,153 (Normalair-Garrett) disclose a flueric partial pressure sensor including a flueric bridge having two bridge legs adapted for sensing a reference gas and a sample-gas mixture. A linear resistor and an orifice resistor are incorporated in each of the bridge legs which are conjoined to discharge from a single outlet. Reference gas and sample-gas mixture are drawn through the respective legs of the bridge by connecting the conjoined outlet to a small flueric aspirator or ejector arranged for operation in the manner of a jet pump by a clean dry gas which may conveniently be the reference gas supplied by way of a pressure reducing valve. Reference gas and sample-gas mixture are supplied to the bridge legs by way of shrouded filters which are open to the same ambient pressure so that pressure difference across each of the bridge legs is the same under all conditions of use. The resistors are arranged to provide an asymmetric balance of the flow rates through the bridge legs, the asymmetric balance being selected so that in operation the bridge output signal, in terms of differential pressure, is constant for a chosen partial pressure of a constituent gas such as oxygen in a sample-gas mixture, for example oxygen-enriched air, in varying absolute pressure conditions such as changes in altitude. Respective pressure signal outlets are connected one with each bridge leg at a position between the linear resistor and the orifice resistor. The bridge output signal is amplified by a flueric laminar flow proportional amplifier which is driven by being connected to the flueric aspirator at the conjoined outlet of the bridge legs. This amplified signal may be used for a number of purposes including control of a switching device or in providing a visual and/or audible warning signal.
An example of use of this flueric partial pressure sensor is the control of a molecular sieve type gas separation system (MSOGS) embodied in an aircraft onboard oxygen generating system (OBOGS) delivering oxygen-enriched air for breathing by aircrew such as is disclosed in EP-A-No. 0,129,304 and corresponding U.S. patent application Ser. No. 595,303 (Normalair-Garrett). The system comprises three molecular sieve beds which in operation are cyclically subjected to a charge/adsorption on-stream phase followed by a purge/desorption regeneration phase. A fixed logic sequencer provides two different overall cycle times and fixes the relative durations of each phase within the overall cycle times. The flueric partial pressure sensor is connected into the system and arranged to receive as its sample-gas mixture a bleed of oxygen-enriched air delivered by the MSOGS. The flueric partial pressure sensor senses the oxygen concentration in the oxygen-enriched air and switches the fixed logic sequencer between the two cycle times so as to obtain an oxygen concentration necessary to maintain the partial pressure of oxygen in the oxygen-enriched air delivered by the MSOGS at a predetermined set point value for the flueric bridge of the sensor irrespective of changes in cabin altitude between sea level and 7600 meters (25000 feet).
The flueric partial pressure sensor has proven completely satisfactory in maintaining oxygen concentration within required minimum and maximum bands through an operational range of aircraft cabin altitude ranging from sea level to 7600 meters (25000 feet) and cabin temperatures ranging from +5.degree. C. to +35.degree. C. However, there is now a requirement to expand the operational temperature range at both the upper and lower ends.
Temperature tests carried out in an altitude chamber have shown the flueric partial pressure sensor to become unstable in operation at temperatures below about -5.degree. C. particularly when operating at altitudes below 2000 meters (7000 feet). These tests have also shown that operation of the amplifier is affected with increasing temperature so that the amplifier gain is progressively reduced particularly under high altitude conditions.
Intensive study, including considerable mathematical analysis, has led towards the conclusion that the problem at the low temperature end results from the flow in the internal flow passages of the amplifier becoming turbulent due to the value of the Reynolds number for the main power jets increasing beyond a critical value of just over 1000 as temperature falls. On the other hand, with increasing temperature and altitude the Reynolds number decreases and tests have shown that the working Reynolds number for the main power jets of the amplifier falls to a value at which the amplifier signal gain is reduced so as to cause malfunctioning of a MSOGS or warning system controlled by the amplifier.